Cryogenics

Photo by: deepspacedave

Cryogenics is the science of producing and studying low-temperature
conditions. The word cryogenics comes from the Greek word
cryos
, meaning "cold," combined with a shortened form of the
English verb "to generate." It has come to mean the
generation of temperatures well below those of normal human experience.
More specifically, a low-temperature environment is termed a cryogenic
environment when the temperature range is below the point at which
permanent gases begin to liquefy. Permanent gases are elements that
normally exist in the gaseous state and
were once believed impossible to liquefy. Among others, they include
oxygen, nitrogen, hydrogen, and helium.

The origin of cryogenics as a scientific discipline coincided with the
discovery by nineteenth-century scientists that the permanent gases can be
liquefied at exceedingly low temperatures. Consequently, the term
"cryogenic" applies to temperatures from approximately
−100°C (−148°F) down to absolute zero (the
coldest point a material could reach).

The temperature of any material—solid, liquid, or gas—is a
measure of the energy it contains. That energy is due to various forms of
motion among the atoms or molecules of which the material is made. A gas
that consists of very rapidly moving molecules, for example, has a higher
temperature than one with molecules that are moving more slowly.

In 1848, English physicist William Thomson (later known as Lord Kelvin;
1824–1907) pointed out the possibility of having a material in
which particles had ceased all forms of motion. The absence of all forms
of motion would result in a complete absence of heat and temperature.
Thomson defined that condition as absolute zero.

Words to Know

Absolute zero:
The lowest temperature possible at which all molecular motion ceases.
It is equal to −273°C (−459°F).

Kelvin temperature scale:
A temperature scale based on absolute zero with a unit, called the
kelvin, having the same size as a Celsius degree.

Superconductivity:
The ability of a material to conduct electricity without resistance. An
electrical current in a superconductive ring will flow indefinitely if a
low temperature (about −260°C) is maintained.

Thomson's discovery became the basis of a temperature scale based
on absolute zero as the lowest possible point. That scale has units the
same size as the Celsius temperature scale but called kelvin units
(abbreviation K). Absolute zero is represented as 0 K, where the term
degree is omitted and is read as zero kelvin. The Celsius equivalent of 0
K is −273°C, and the Fahrenheit equivalent is
−459°F. One can convert between Celsius and Kelvin scales by
one of the following equations:

One aspect of cryogenics involves the development of methods for producing
and maintaining very low temperatures. Another aspect includes the study
of the properties of materials at cryogenic temperatures. The mechanical
and electrical properties of many materials change very dramatically when
cooled to 100 K or lower. For example, rubber, most plastics, and some
metals become exceedingly brittle. Also many metals and ceramics lose all
resistance to the flow of electricity, a phenomenon called
superconductivity. In addition, helium that is cooled to very nearly
absolute zero (2.2 K) changes to a state known as superfluidity. In this
state, helium can flow through exceedingly narrow passages with no
friction.

History

Cryogenics developed in the nineteenth century as a result of efforts by
scientists to liquefy the permanent gases. One of the most successful of
these scientists was English physicist Michael Faraday (1791–1867).
By 1845, Faraday had managed to liquefy most permanent gases then known to
exist. His procedure consisted of cooling the gas by immersion in a bath
of ether and dry ice and then pressurizing the gas until it liquefied.

Six gases, however, resisted every attempt at liquefaction and were known
at the time as permanent gases. They were oxygen, hydrogen, nitrogen,
carbon monoxide, methane, and nitric oxide. The noble gases—helium,
neon, argon, krypton, and xenon—were yet to be discovered. Of the
known permanent gases, oxygen and nitrogen (the primary constituents of
air), received the most attention.

For many years investigators labored to liquefy air. Finally, in 1877,
Louis Cailletet (1832–1913) in France and Raoul Pictet
(1846–1929) in Switzerland succeeded in producing the first
droplets of liquid air. Then, in 1883, the first measurable quantity of
liquid oxygen was produced by S. F. von Wroblewski (1845–1888) at
the University of Krakow. Oxygen was found to liquefy at 90 K, and
nitrogen at 77 K.

Following the liquefaction of air, a race to liquefy hydrogen ensued.
James Dewar (1842–1923), a Scottish chemist, succeeded in 1898. He
found the boiling point of hydrogen to be a frosty 20 K. In the same year,
Dewar succeeded in freezing hydrogen, thus reaching the lowest temperature
achieved to that time, 14 K. Along the way, argon was discovered (1894) as
an impurity in liquid nitrogen. Somewhat later, krypton and xenon were
discovered (1898) during the fractional distillation of liquid argon.
(Fractional distillation is accomplished by liquefying a mixture of
gases, each of which has a different boiling point. When the mixture is
evaporated, the gas with the highest boiling point evaporates first,
followed by the gas with the second highest boiling point, and so on.)

Each of the newly discovered gases condensed at temperatures higher than
the boiling point of hydrogen but lower than 173 K. The last element to be
liquefied was helium gas. First discovered in 1868 in the spectrum of the
Sun and later on Earth (1885), helium has the lowest boiling point of any
known substance. In 1908, Dutch physicist Heike Kamerlingh Onnes
(1853–1926) finally succeeded in liquefying helium at a temperature
of 4.2 K.

Methods of producing cryogenic temperatures

Cryogenic conditions are produced by one of four basic techniques: heat
conduction, evaporative cooling, cooling by rapid expansion (the
Joule-Thomson effect), and adiabatic demagnetization. The first two are
well known in terms of everyday experience. The third is less well known
but is commonly used in ordinary refrigeration and air conditioning units,
as well as in cryogenic applications. The fourth process is used primarily
in cryogenic applications and provides a means of approaching absolute
zero.

Cryotubes used to store strains of bacteria at low temperature.
Bacteria are placed in little holes in the beads inside the tubes and
then stored in liquid nitrogen.
(Reproduced by permission of

Photo Researchers, Inc.

)

Heat conduction is a relatively simple concept to understand. When two
bodies are in contact, heat flows from the body with the higher
temperature to the body with a lower temperature. Conduction can occur
between any and all forms of matter, whether gas, liquid, or solid. It is
essential in the production of cryogenic temperatures and environments.
For example, samples may be cooled to cryogenic temperatures by immersing
them directly in a cryogenic liquid or by placing them in an atmosphere
cooled by cryogenic refrigeration. In either case, the sample cools by
conduction (or transfer) of heat to its colder surroundings.

The second process for producing cryogenic conditions is evaporative
cooling. Humans are familiar with this process because it is a mechanism
by which our bodies lose heat. Atoms and molecules in the gaseous state
are moving faster than atoms and molecules in the liquid state. Add heat
energy to the particles in a liquid and they will become gaseous. Liquid
perspiration on human skin behaves in this way. Perspiration absorbs body
heat, becomes a gas, and evaporates from the skin. As a result of that
heat loss, the body cools down.

Cryogens and Their Boiling Points

Boiling Point

Cryogen

°F

°C

K

Oxygen

−297

−183

90

Nitrogen

−320

−196

77

Hydrogen

−423

−253

20

Helium

−452

−269

4.2

Neon

−411

−246

27

Argon

−302

−186

87

Krypton

−242

−153

120

Xenon

−161

−107

166

In cryogenics, a container of liquid is allowed to evaporate. Heat from
within the liquid is used to convert particles at the surface of the
liquid to gas. The gas is then pumped away. More heat from the liquid
converts another surface layer of particles to the gaseous state, which is
also pumped away. The longer this process continues, the more heat is
removed from the liquid and the lower its temperature drops. Once some
given temperature is reached, pumping continues at a reduced level in
order to maintain the lower temperature. This method can be used to
reduced
the temperature of any liquid. For example, it can be used to reduce the
temperature of liquid nitrogen to its freezing point or to lower the
temperature of liquid helium to approximately 1 K.

A third process makes use of the Joule-Thomson effect, which was
discovered by English physicist James Prescott Joule (1818–1889)
and William Thomson, Lord Kelvin, in 1852. The Joule-Thomson effect
depends on the relationship of volume (bulk or mass), pressure, and
temperature in a gas. Change any one of these three variables, and at
least one of the other two (or both) will also change. Joule and Thomson
found, for example, that allowing a gas to expand very rapidly causes its
temperature to drop dramatically. Reducing the pressure on a gas
accomplishes the same effect.

To cool a gas using the Joule-Thomson effect, the gas is first pumped into
a container under high pressure. The container is fitted with a valve with
a very small opening. When the valve is opened, the gas escapes from the
container and expands quickly. At the same time, its temperature drops.
The first great success for the Joule-Thomson effect in cryogenics was
achieved by Kamerlingh Onnes in 1908 when he liquefied helium.

The Joule-Thomson effect is an important part of our lives today, even
though we may not be aware of it. Ordinary household refrigerators and air
conditioners operate on this principle. First, a gas is pressurized and
cooled to an intermediate temperature by contact with a colder gas or
liquid. Then, the gas is expanded, and its temperature drops still
further. The heat needed to keep this cycle operating comes from the
inside of the refrigerator or the interior of a room, producing the
desired cooling effect.

The fourth process for producing cryogenic temperatures uses a phenomenon
known as adiabatic demagnetization. Adiabatic demagnetization makes use of
special substances known as paramagnetic salts. A paramagnetic salt
consists of a very large collection of particles that act like tiny
(atom-sized) magnets. Normally these magnetic particles are spread out in
all possible directions. As a result, the salt itself is not magnetic.
That condition changes when the salt is placed between the poles of a
magnet. The magnetic field of the magnet causes all the tiny magnetic
particles in the salt to line up in the same direction. The salt becomes
magnetic, too.

At this exact moment, however, suppose that the external magnet is taken
away and the paramagnetic salt is placed within a liquid. Almost
immediately, the tiny magnetic particles within the salt return to their
random, every-which-way condition. To make this change, however, the
particles require an input of energy. In this example, the energy is
taken from the liquid into which the salt was placed. As the liquid gives
up energy to the paramagnetic salt, its temperature drops.

Adiabatic demagnetization has been used to produce some of the coldest
temperatures ever observed—within a few thousandths of a degree
kelvin of absolute zero. A related process involving the magnetization and
demagnetization of atomic nuclei is known as nuclear demagnetization. With
nuclear demagnetization, temperatures within a few millionths of a degree
of absolute zero have been reached.

Applications

Following his successful liquefaction of helium in 1908, Kamerlingh Onnes
turned his attention to the study of properties of other materials at very
low temperatures. The first property he investigated was the electrical
resistance of metals. Electrical resistance is the tendency of a substance
to prevent the flow of an electrical current through it. Scientists had
long known that electrical resistance tends to decrease with decreasing
temperature. They assumed that resistance would completely disappear at
absolute zero.

Research in this area had great practical importance. All electrical
appliances (ovens, toasters, television sets, and radios, for example)
operate with low efficiency because so much energy is wasted in overcoming
electrical resistance. An appliance with no electrical resistance could
operate at much less cost than existing appliances.

What Onnes discovered, however, was that for some metals, electrical
resistance drops to zero very suddenly at temperatures above absolute
zero. The effect is called superconductivity and has some very important
applications in today's world. For example, superconductors are
used to make magnets for particle accelerators (devices used, among other
things, to study subatomic particles such as electrons and protons) and
for magnetic resonance imaging (MRI) systems (a diagnostic tool used in
many hospitals).

The discovery of superconductivity led other scientists to study a variety
of material properties at cryogenic temperatures. Today, physicists,
chemists, material scientists, and biologists study the properties of
metals, as well as the properties of insulators, semiconductors, plastics,
composites, and living tissue. Over the years, this research has resulted
in the identification of a number of useful properties. One such property
common to most materials that are subjected to extremely low temperatures
is brittleness. The recycling industry takes advantage of this by
immersing
recyclables in liquid nitrogen, after which they are easily pulverized
and separated for reprocessing.

Still another cryogenic material property that is sometimes useful is that
of thermal contraction. Materials shrink when cooled. To a point (about
the temperature of liquid nitrogen), the colder a material gets the more
it shrinks. An example is the use of liquid nitrogen in the assembly of
some automobile engines. In order to get extremely tight fits when
installing valve seats, for instance, the seats are cooled to liquid
nitrogen temperatures, whereupon they contract and are easily inserted in
the engine head. When they warm up, a perfect fit results.

Cryogenic liquids are also used in the space program. For example,
cryogenic materials are used to propel rockets into space. A tank of
liquid hydrogen provides the fuel to be burned and a second tank of liquid
oxygen is provided for combustion.

Another space application of cryogenics is the use of liquid helium to
cool orbiting infrared telescopes. Infrared telescopes detect objects in
space not from the light they give off but from the infrared radiation
(heat) they emit. However, the operation of the telescope itself also
gives off heat. What can be done to prevent the instrument from being
blinded by its own heat to the infrared radiation from stars? The answer
is to cool parts of the telescope with liquid helium. At the temperature
of liquid helium (1.8 K) the telescope can easily pick up infrared
radiation of the stars, whose temperature is about 3 K.

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